Techniques to control the amplitude and phase of semiconductor laser diodes promise to extend the performance of laser diode systems and thereby enable a wide range of new applications. Presently, semiconductor lasers exhibit several favorable attributes, including low cost, small size and high electrical efficiency, which have made them key elements in communications systems and high power laser systems (e.g., for pumping fiber lasers). One limitation of the latter application, however, is that as the optical power of semiconductor diode lasers and fiber lasers increases, several factors begin to degrade the spectral and spatial quality of the optical output beam. For example, thermal non-uniformities such as filamentation produce spatially and spectrally multi-moded outputs with poor beam quality for semiconductor diode lasers above ˜1 Watt. In optical fiber-based systems, optical nonlinearities such as stimulated Brillioun scattering (SBS) lead to spectral broadening and dynamic instabilities for power levels ˜1 kW. Each of these factors can seriously degrade the optical performance of typical implementations of a high power laser. Therefore, for applications requiring near-diffraction limited beams with good focusability and low divergence, it challenging to exploit the advantages of diode laser-based systems and scale their power beyond 1 KW.
Much progress has been made in the development of Ytterbium (Yb), Erbium (Er) and Prasedymium (Pr)-doped fiber lasers and amplifiers utilizing semiconductor diode lasers. These semiconductor pump diodes are fiber coupled to a large diameter multimode core, co-linear with a single mode doped core in which a lasing mode or amplified seed signal propagates. The multimode pump power spatially overlaps and is absorbed within the central, single mode core, such that pump energy at a wavelength of 900 nm to 975 nm for Yb fiber amplifiers is efficiently transferred (>80%) to a single optical mode within the 1055 nm to 1075 nm spectral range. Since the optical radiation is confined within the single mode waveguide core, the optical intensities are very high and can produce optical non-linearities. The intensity can readily exceed the stimulated Brillioun scattering (SBS) threshold, which is typically about 10 W for 10 meters of single mode, polarization-maintaining (PM) fiber with a 6-micron core diameter at 1064 nm. Above this threshold, several negative factors conspire to degrade or limit laser performance. Primarily, the optical spectrum broadens and significant light is coupled into the frequency-shifted, SBS backscattered light.
In the prior art, the degradation of the laser's spectral and spatial modes are avoided by maintaining power levels below about a few 100's of W for large mode area, single mode PM fiber amplifiers and/or lasers. This is accomplished by increasing the central core diameter and by decreasing the numerical aperture by a compensating amount to remain single mode or near-single mode. Introducing index of refraction non-uniformities along the length of active fiber, by way of longitudinal temperature and/or strain gradients, for example, mitigates SBS effects.
To scale to high power levels, it is desirable to combine the outputs of multiple, intermediate power fiber amplifiers or fiber lasers. To accomplish this, a single “master” fiber laser is split into a number of branches which seed a corresponding number of phase modulators and fiber amplifiers. U.S. Pat. No. 5,946,130 by Rice discloses an array of optical fiber amplifiers seeded by a common master laser whose output is power split, amplified, and coherently re-combined by use of phase shifters and control circuits to remove the uncorrelated phase errors introduced by the separated amplifier paths.
Betin et al. have disclosed an alternate approach to coherent combining of fiber amplifiers in U.S. Patent Application US2006/0078033A1. In this application, physically separate but optically coupled fiber lasers are forced to lase at the same frequency by optically injection-locking each fiber laser with the optical outputs of other fiber lasers.
Other techniques to combine the multiple coherent outputs into a single diffraction limited optical beam have been disclosed. U.S. Pat. No. 5,307,369 by Kimberlin describes the use of optical beam splitting plates to coherently combine the outputs of a plurality of laser. U.S. Pat. No. 4,813,762 by Leger et al. describes the use of diffractive lenslet arrays within a laser array cavity to aperture fill and coherently combine the outputs. U.S. Patent Application 2007/0086010 by Rothenberg describes a technique for interferometric beam combining of multiple beams into a single composite output beam.
In our previously filed patent application US2006/239312A1, we have disclosed two-dimensional semiconductor laser arrays which are individually driven by electronic phase-locked loops, wherein the loop controller locks the frequency and phase of each high power laser element to the same reference laser by way of a low delay electronic feedback path. Two lasers which are locked thereby exhibit a high degree of mutual coherence and can be coherently combined. This approach can produce a single frequency, single spatial mode output beam of high quality with high electrical efficiency.
An important aspect of future high power laser systems is their ability to scale to optical powers in excess of 10's of KW. This scalability is impacted, for example, by design tradeoffs resulting from heat extraction requirements as well as optical nonlinearities and non-idealities. Practical issues such as alignment tolerances, reliability, electrical efficiency, cost, size and weight are also key factors influencing the successful realization of high power laser systems. Techniques to extend the performance of laser systems from a power and also linewidth point of view are necessary. A key aspect of such applications is the need to control the coherence and noise characteristics of semiconductor diode laser elements. The techniques to provide such control are based on optical phase-locked loops (OPLLs). High power laser systems are only one example of the many applications which benefit from advanced techniques to phase-lock semiconductor laser diodes.